Colloidal gold in aqueous solution (gold sols) is well-known. A reasonably good understanding of these sols exists, and they have found uses in ceramics, medicine, and other areas. Gold sols are usually prepared by reduction of halide salts, such as HAuCl.sub.4. The gold particles and the by-products of the reduction remain in the aqueous solution The colloidal particles are stabilized because of the charged double layer which is dependent on adsorbed anions, usually chloride, that remain in solution after the reduction process. See Jirgensons, B.; Straumanis, M. E., "Colloid Chemistry"; MacMillan: New York, 1962: pp. 119, 130, 258, 306.
Faraday published the first scientific investigations of gold sols. Phil. Trans. (1857) 147, 145. He usually reduced aqueous solutions of AuCl.sub.3 with phosphorous. However, he also experimented with sparking gold wires. He concluded that gold was present in the solutions as elemental gold, and that color depended on particle size. He also discovered flocculation by addition of an electrolyte NaC1. Other workers also used sparking of metals under water to produce sols of Pb, Sn, Au, Pt, Bi, Sb, As, Tl, Ag, and Hg. See Mindel, et al., J. Am. Chem. Soc. (1943), 65:2112, and refs. cited.
The preparation of colloidal metals in nonaqueous media has received less attention. Kurihara et al., J. Am. Chem. Soc. (1983), 105:2574-2579, have reported on gold colloids in water-in-oil emulsions Water-polymer-solvent systems have also been reported: Ledwith, A., Chem. Ind. (London) 1956, 1310. Blumencron, W., Med. Monatsschr. 1957, 11, 89.
Another approach in preparing and stabilizing metal colloids is by macromolecule adsorption-stabilization. A wide variety of materials has been used including gummy gelatinous liquids, albumin, Icelandic moss, latex, polyvinylpyrrolidone, antibodies, carbowax 20M, polyvinylpyridine, and various polymer-water/oil-water mixtures. These studies clearly indicate that "steric stabilization" of metal colloids is also important (along with electronic stabilization). See Hirtzel, et al., "Colloidal Phenomena: Advanced Topics," Noyes Pubs., N.J., 1985, pp. 88-97.
Methods and apparatus are known for generating atomic metal vapors under high vacuum and capturing the vaporized metal in organic solvents. A prototype apparatus which can be used for this purpose is described in Klabunde, et al. (1979), Inorg. Synth. 19:59-86. The Klabunde apparatus may be used for organic chemical reactions with metal vapors: Klabunde (1975), Accounts of Chemical Research, 8(12):393-399. The apparatus has also been used to form slurries of solvated metals and to prepare catalysts. See Klabunde and Murdock (1979), J. Org. Chem., 44:3901-3908; Matsuo and Klabunde (1982), J. Org. Chem., 47:843-848; and Klabunde and Tanaka (1983), J. Mol. Catal., 21:57-79. Metal slurries were formed by warming a frozen matrix of solvent and dispersed metal, the metal atoms clustering and forming precipitated solids during warming. For catalyst preparation, precipitation of solvated metal was carried out in the presence of catalyst supports, thereby depositing the metals on the supports. The Klabunde apparatus and methods used therewith have not previously been known to be capable of producing stable colloidal dispersions. On the contrary it was believed, as stated in Klabunde and Tanaka (1983), cited above, at page 59: "Clustering of metal atoms in organic media invariaby leads to metal powders that are pseudoorganometallic in nature".
Kimura and Bandow have reported trying to prepare metal colloids in organic solvents by several methods: Bull. Chem. Soc. Jpn. (1983), 56:3578-3584. One procedure referred to as the "gas flow-solution trap method" was reported as achieving stable colloidal dispersions from several metals using ethanol as the suspending solvent. Kimura and Bandow sought to produce their dispersions for the purpose of studying the individual colloidal metal particles. To prepare micrographs of the particles, specimens of their dispersions were dropped onto a "Cu grid coated by colloidal film reinforced by carbon film" (page 3579, col. 2, lines 2-3). They wanted to view and measure the separate particles, as illustrated by the micrographs reproduced in their report. No practical use of colloidal dispersions was indicated.
As far as is known, there is no teaching, or even a suggestion, that continuous metal coatings can be formed from organic solvent dispersions of colloidal metal particles; more direct and successful methods for the preparation of nonaqueous metal sols are recognized as desirable. Especially valuable would be methods that avoid: (1) the metal salt reduction step, and thus prevent contamination by other reagents; (2) electrical discharge methods which decompose organic solvents; and (3) the need for macromolecule stabilization. Such a method would provide pure, non-aqueous metal colloids and should make efficient use of precious metals employed. Such colloids would be valuable technologically in many ways.